Adaptive filtering is used in a variety of situations, including power amplifier predistortion systems. Predistortion is a technique which improves the linearity of power amplifiers, for example in telecommunications systems. A power amplifier with nonlinearities causes interference on other radio channels. Predistortion circuits inversely model the power amplifier's gain and phase characteristics to produce a system that is more linear. Inverse distortion is introduced into the input of the power amplifier, cancelling nonlinearities in the amplifier. The characteristics of the adaptive filter may vary by the type of power amplifier or power amplifier sub-system architecture.
An adaptive equalizer provides feedback, for example to equalize the channel gain across frequency bandwidths to compensate for different gains at different frequencies. Also, adaptive filters may be used for interference calculations in other types of adaptive systems. An adaptive filter self-adjusts its transfer function based on an optimization algorithm from an error signal. In an example, an adaptive process uses a cost function, which is a criterion for optimum performance of the filter, as an input to an optimization algorithm. The algorithm determines how to modify the filter transfer function to minimize the cost of the next iteration.